One method to improve fuel economy and reduce emissions in an engine with electromechanical valves is to operate the engine with a reduced number of firing cylinders per unit time for some operating conditions. By reducing the number of firing cylinders, the engine pumping losses can be reduced. When transitioning between different numbers of firing cylinders, air charge, spark timing, and fuel can be adjusted on a cylinder-by-cylinder basis to deliver the desired engine torque and provide smooth torque output.
However, when the number or firing rate of the firing cylinders is changed, a transient in the individual cylinder engine output torque may be generated, even if the cycle average torque before and after the transition is maintained relatively constant. Such transient operation may excite driveline/vehicle vibration modes, such as a shuffle mode.
One approach to suppress the vibration of the vehicle due to variation of engine output when reactivating cylinders is described in U.S. Pat. No. 5,597,371. In the '371 reference, a timing is detected according to which a vehicle accelerates from a fuel cut state, and a cylinder is detected in which combustion takes place immediately after this timing. A cylinder corresponding to a predetermined combustion order from the detected cylinder is then specified. By reducing the torque produced in the specified cylinder, forward/backward vibration of the vehicle due to variation of engine output torque during acceleration from coasting (no fuel firing) is allegedly suppressed.
However, the inventors have recognized several disadvantages with the above approach. As one example, the inventors herein have recognized that even if torque is reduced as described above, the vibration mode may still be excited depending on engine speed, the number of cylinders deactivated, and other factors. As another example, the inventors have recognized that decreasing engine torque during the transition can exacerbate vibration in some conditions. Finally, the inventors herein have recognized that vehicle vibration may also occur when deactivating engine cylinders or when a torque per cylinder in the first operating mode may be greater than the torque per cylinder in the second operating mode or when the number of the firing cylinders in the first operating mode is less than the number of the firing cylinders in the second operating mode. Again, in these cases, the '371 approach may actually exacerbate vehicle vibration.